Inheritage from ndarray calls __getitem__

Question

Hi I'm trying to derive a class from ndarray. I'm sticking to the recipe found in docs but I get an error I do not understand, when I override a __getiem__() function. I'm sure this is how it is supposed to work but I do not understand how to do it correctly. My class that basically adds a "dshape" property looks like:

class Darray(np.ndarray):
    def __new__(cls, input_array, dshape, *args, **kwargs):
        obj = np.asarray(input_array).view(cls)
        obj.SelObj = SelObj
        obj.dshape = dshape
        return obj

    def __array_finalize__(self, obj):
        if obj is None: return
        self.info = getattr(obj, 'dshape', 'N')  

    def __getitem__(self, index):        
        return self[index]

when I now try to do:

D = Darray( ones((10,10)), ("T","N"))

the interpreter will fail with a maximum depth recursion, because he calls __getitem__ over and over again.

can someone explain to me why and how one would implement a getitem function?

cheers, David

Answer 1

can someone explain to me why and how one would implement a getitem function?

For your current code, a __getitem__ isn't needed. Your class works fine (except for the undefined SelObj) when I remove the __getitem__ implementation.

The reason for the maximum recursion depth error is the definition of __getitem__, which uses self[index]: a shorthand notation for self.__getitem__(index). If you must override __getitem__, then make sure you call the superclass implementation of __getitem__:

def __getitem__(self, index):
    return super(Darray, self).__getitem__(index)

As for why you'd do this: there are lots of reasons for overriding this function, e.g. you might associate names with the rows of an array:

class NamedRows(np.ndarray):
    def __new__(cls, rows, *args, **kwargs):
        obj = np.asarray(*args, **kwargs).view(cls)
        obj.__row_name_idx = dict((n, i) for i, n in enumerate(rows))
        return obj

    def __getitem__(self, idx):
        if isinstance(idx, basestring):
            idx = self.__row_name_idx[idx]
        return super(NamedRows, self).__getitem__(idx)

Demo:

>>> a = NamedRows(["foo", "bar"], [[1,2,3], [4,5,6]])
>>> a["foo"]
NamedRows([1, 2, 3])

Answer 2

The problem is here:

def __getitem__(self, index):        
    return self[index]

foo[index] just calls foo.__getitem__(index). But in your case, that just returns foo[index], which just calls foo.__getitem__(index). Which repeats in an infinite loop until you run out of stack space.

If you want to defer to your parent class, you have to do this:

def __getitem__(self, index):        
    return super(Darray, self)[index]

… or, maybe more explicitly:

def __getitem__(self, index):        
    return super(Darray, self).__getitem__(index)